As a medical endoscope, it is preferable to acquire an image of a high resolution and a wide angle of observation field, while ensuring low invasive to the patient. However, high image quality and less invasive are opposite to each other. Specifically, in order to obtain a high quality image, it is necessary to increase the number of pixels, and it is preferable to use a large imaging element. On the other hand, as the size of the imaging element increases, the imaging lens diameter becomes large, and as a result, the external diameter becomes larger, it makes it difficult to ensure the less invasiveness.
Therefore, in recent years, the pixel pitch is made smaller, the number of pixels is increased without increasing the size of the imaging element, a method for obtaining high quality images are becoming the mainstream. The pixel pitch has been becoming smaller and smaller as time goes and imaging elements having a pixel pitch of a few microns or less have also been developed.
For example, Patent Literature 1 and Patent Literature 2 disclose an objective optical system adapted to the miniaturized imaging element with the above-described pixel pitch which is several microns or less.